Lime-induced growth depression in Lupinus species: Are soil pH and bicarbonate involved?

In a series of experiments, the effect of elevated pH on root growth of dif ferent lupin genotypes and lime-tolerant Pisum sativum was assessed. Seedli ngs were cultivated in pH-controlled but unbuffered nutrient or test soluti ons with pH 5-8 under constant light or constant darkness. Moderately incre ased solution pH did not affect root elongation. At high pH, root elongatio n rates were reduced by less than 35%, L. luteus being most sensitive, in a further experiment, effects of 10 mM bicarbonate (HCO3-) and organic buffe rs (MES, TES) were examined. Bicarbonate severely affected root elongation of lupins, sensitivity increasing in the order Lupinus albus < L. angustifo lius < L. luteus. Since the reduced root elongation was not accompanied by a decreased root weight, specific root weight was increased. Shoot growth w as not affected by HCO3-. Addition of organic buffers to alkaline nutrient solution, similarly to HCO3-, reduced elongation of main roots, but not tha t of laterals. Both treatments increased citrate concentrations in roots of sensitive L. luteus, but not of tolerant L. albus. Other carboxylates were not related to growth inhibition. Xylem exudate pH was always below 5.8, i ndicating that HCO3- was probably not translocated to the shoot. It is conc luded that HCO3- is a key factor for lime-induced growth inhibition of calc ifuge lupins, with the roots, not the shoots, being primarily affected.